Compound semiconductor devices are divided into various types. Among them, a high electron mobility transistor (HEMT) has advantages of low noise and high-speed operation, and is used in microwave and millimeter-wave amplifiers for these advantages.
Since the channel layer of an HEMT is not doped with impurities, no impurity scattering occurs in the channel layer, whereby the mobility of two-dimensional electron gas in the channel layer increases.
There are a variety of combinations of the materials for the channel layer and a compound semiconductor substrate.
For example, there is a GaAs HEMT in which an i-type InxGa1-xAs (x=0.1 to 0.2) layer is formed as a channel layer on a GaAs substrate. Such a GaAs HEMT is called a pseudomorphic HEMT (p-HEMT), because the GaAs substrate and the channel layer are pseudo-lattice-matched. In the p-HEMT, In in InxGa1-xAs of the channel layer contributes to further increase in the mobility of two-dimensional electron gas.
There is also an InP HEMT, in which an InP substrate is used as a compound semiconductor substrate. The channel layer of an InP HEMT is, for example, an i-type In0.53Ga0.47As layer which is lattice-matched with the InP substrate. For the purpose of increasing the mobility of electrons, an i-type InxGa1-xAs (0.53<x) layer with a higher content ratio of In than the In0.53Ga0.47As layer is used as the channel layer of the InP HEMT in some cases.
It was reported that, when an InGaAs layer is reduced in temperature, two-dimensional electron gas is less likely to suffer phonon scattering and therefore increases in mobility. For instance, it was reported that the mobility of two-dimensional electron gas in an In0.53Ga0.47As layer, which is 10,000 cm2/Vs at room temperature (300K), improves to 60,000 cm2/Vs at extremely low temperature (4K).
Since mobility in the channel layer of the HEMT thus improves at low temperature, it is contemplated that the HEMT can be used to achieve an amplifier producing low noise at low temperature.
Noted that the related techniques are disclosed in the following literature.
Japanese Laid-open Patent Publication No. 2008-98674;
Japanese Examined Laid-open Patent Publication No. Hei 5-4812;
Matsuoka et al., “Temperature Dependence of Electron Mobility in InGaAs/InAlAs Heterostructures”, Japanese Journal of Applied Physics. vol. 29, no. 10, pp. 2017-2025, 1990;
Akazaki et al., “Kink Effect in an InAs-Inserted-Channel InAlAs/InGaAs Inverted HEMT at Low Temperature”, IEEE Electron Device Letters., vol. 17, no. 7, pp. 378-380, 1996; and
Oliver et al., “ELECTRICAL CHARACTERIZATION AND ALLOY SCATTERING MEASUREMENTS OF LPE GaxIn1-xAs/InP FOR HIGH FREQUENCY DEVICE APPLICATIONS”, Journal of Crystal Growth 54, pp. 64-68, 1981.